It is known to provide earphones which may be inserted into a user's ear cavity or headphones comprising a small loudspeaker mounted on a headband and arranged to be placed against or over the user's ear. Such sound sources transmit sound to a user's inner ear via the ear drum using air pressure waves passing along the ear canal.
A typical conventional earphone uses a moving coil type transducer mounted in a plastic housing. The moving coil is connected to a light diaphragm which is designed to fit into the entrance of the ear canal. The moving coil and diaphragm are light and are coupled intimately to the eardrum at the other end of the ear canal. The acoustic impedance of the eardrum and ear canal seen by the moving coil transducer is relatively small. This small impedance in conjunction with the intimate coupling means that the motion requirements of the moving coil transducer are relatively low.
A moving coil transducer requires a magnetic circuit, which typically contain metal parts, e.g. steel or iron pole pieces, to generate magnetic field lines for the coil to move. These parts provide a relatively large inertial mass which combined with the low motion requirement means that relatively little vibration enters the housing.
There are disadvantages associated with both headphones and earphones. For example, they may obstruct normal auditory process such as conversation or may prevent a user from hearing useful or important external audio information, e.g. a warning. Furthermore, they are generally uncomfortable and if the volume of the sound being transmitted is too high they may cause auditory overload and damage. Earpieces which fit into the ear canal also have hygiene issues.
An alternative method of supplying sound to a user's inner ear is to use bone conduction as for example in some types of hearing aids. In this case, a transducer is fixed to a user's mastoid bone to be mechanically coupled to the user's skull. Sound is then transmitted from the transducer through the skull and directly to the cochlea or inner ear. The eardrum is not involved in this sound transmission route. Locating the transducer behind the ear provides good mechanical coupling. Nevertheless, quite high power and applied force is generally necessary for good results.
One disadvantage is that the mechanical impedance of the skull at the location of the transducer is a complex function of frequency. Furthermore, the apparatus needs to be a favourable fit on the skill. Thus, the design of the transducer and the necessary electrical equalisation may be expensive and difficult.
Alternative solutions are proposed in JP56-089200 (Matsushita Electric Ind Co Ltd), WO 01/87007 (Temco Japan Co, Ltd) and WO 02/30151 and WO 05-025267 to the present applicant. In each publication, a transducer is coupled direct to a user's pinna, in particular behind a user's earlobe, to excite vibration therein whereby an acoustic signal is transmitted to the user's inner ear.
WO 02/30151 and WO 05-025267 describe various ways of attaching a transducer to a user's pinna, including specially designed hooks and clips.